Xerographic color masking



July 10, 1962 .1. T. BICKMORE XEROGRAPHIC COLOR MASKING Filed July 8, 1958 INVENTORQ John T. Bickmore 5% QOQQ United States Patent G 3,043,686 XERQQRAPHIC CGLOR MASKING John T. Bickmore, Rochester, N.Y., assignor to Xerox Corporation, a corporation of New York Filed July 8, 1958, Ser. No. 747,256

. 1 Claim. (Cl. 96-1) vidual imagebeing inversely proportional to the brightness of its complementary color in the original scene. This procedure can be carried out in the art of photography by makingred, green and blue color separation negatives from an original scene or a color transparency and using each separation negative to form its COl'l'fi spending, color image or color printing plate through such well known photomechanical processes as the dye transfer process, photolithography or photoengraving.

7 Corresponding techniques can also be used in the art of xerography, which is an electrostatic image reproduction process, for color print making. Typically, in xerography as disclosed in US. Patents 2,297,691, 2,573,881, 2,576,047, 2,600,580, 2,618,551, 2,618,552, 2,619,418, 2,638,416, 2,659,670, 2,690,394, 2,725,304, 2,745,327, 2,751,616, 2,772,991, 2,777,957, 2,777,418, 2,808,023, 2,815,330, 2,817,598, 2,817,765 and others, a xerographic plate which includes a photoconductive insulating layer is electrostatically charged, exposed to a light pattern so as to selectively discharge. the plate andform an electro static latent image and the latent image is then developed by the selective deposition of electrostatically attractable particles to form a visible image which may then be viewed on the xerographic plate or be transferred to another surface. In conventional xerographic color print making threeseparate images aremade on the same or different xerographic plates, each image corresponding to a different primary color. Thus, the xerographic plates can be exposed through red, green and blue filters or to red, green and blue color separation transparencies. The three resulting images may be developed with cyan, magenta and yellow particles which may be transferred to a single support to form a color image, or by other well known xerographic techniques, each developed image may be employed to form a separate printing plate and the printing plates may then be used in a conventional multicolor press to forin a plurality of prints.

Both the photographic and xerographic techniques thus far described are incapable of producing color prints of really high quality because of inherent limitations in available cyan, magenta and yellow inks, dyes or pigments which must be used in any color printing process to form the final image. An ideal cyan should absorb only red light, magenta should absorb only green light, and yellow should absorb only blue. vIn practice, however, it is found that while available yellow colors are'reasonably close to the ideal, cyans absorb heavily in the green and to a lesser extent in the blue, and magentas absorb heavily in the blue These deficiencies in availablecolors result in a serious degradation of color image quality when the above described print making techniques are used. In commercial color printing these deficiencies are largely compensated for by techniques known as masking in which the density of a printing color, i.e. cyan, magenta or yellow, is made to depend not only on the brightness of its complementary color in the original scene, but also to a lesser extent on the'brightness of one or. both of the other primary colors. In one method of masking ice often used in photography a red color separation positive is made and bound in register with the green separation negative. This combination is then used to form the magenta dye image and the magenta printing plate. It can be seen that the density of the magenta color,'which is intended primarily to absorb green, is thus decreased in those areas of the print Where the cyan color occurs. It will be noted that the'cyan color which is formed from the red separation negative has an unwanted absorption in the green part of the spectrum. In this Way, the total green absorption of the printis'made to more faithfully correspond to the amount of green in the original scene. Similarly, a weak green color. separation positive is made, bound in register with the blue separation negative and the combination used to form the yellow color. Thus, in areas where the magenta color isintense and contributes unwanted blue absorption the intensity of the yellow color is correspondingly decreased.

These masking techniques can also be adapted to xerographic color printmaking. They are, however, very however, we have discovered a color maskingtechnique specifically adaptedto xerography which greatly simplifies the making of color prints. I

The invention will be described in connection with the drawing in which:

- FIG. 1 represents .a simplified schematic perspective view of an apparatus suitable for carrying out the invention; v 3

FIG. 2 represents an enlarged perspective view of part of the apparatus of FIG. 1. e

In FIG. 1, 4 is a conventional photographic'enlarger for projecting a transparency such as a color transparency or a color separation positive onto a xerographic plate (not shown) located in charging apparatus 5.

FIG. 2 shows. the construction of charging apparatus 5 as shown in FIG. 1. Supportframe 10 serves to position all components and includes abase plate 11 generally made of conductive material but which can optionally be-made of glass or the like upon which is placed xerographic plate 12 comprising a photoconductive insulating layer '13 and a conductive support 14. In some cases conductive support 14 may be omitted. Conductive rails Y17 and 18 are positioned on opposite sides 'of frame 10" and each rail is secured to' frame 10 but insulated-therefrom by insulator 19. A chargi'ngbar or scorotron 21 is slidably mounted in frame 10 and may be passed over xerographic plate 12 at a slight distance therefrom. It comprises a support channel 22 generally of conductive material, car: rying at each end insulating blocks 23 between which are a set of three corona generating Wires 35 and a set of screen wires '36. Charging bar 21 is supported in frame 10 by hangers 25 and 26 which slide on rails 17 and 18 respectively. Hanger 25 is connected to corona generating wires 35 and hanger 26 is connected to screen wires 36 whereby the potential of each set of wires may be controlled byiapplying a potential to the corresponding rail. High voltage power supply 30 is connected to base plate 11 and thus to conductive support 14 through wire 31 and is connected to the two sets of wires in charging bar 21 through wires 33 and 34 connected to rails 17 and 18 respectively. Frame 10 also rotatably'supports a reversing lead screw 27 which is adapted to move the charging bar back and forth over xerographic plate 12 by means of .the lead screw block 28 which engages the groove in the lead screw and is attached to support channel 22. Electric motor 29, also attached to frame 10, is used to rotate lead screw 27. Conventional control apparatus such as a microswitch (not shown) may be provided to stop motor 29 in-charging bar 21 after charging bar 21 passes back and forth over xerographic plate 12 and returns to the parent conductive electrode 39.

starting position as shown. Frame also includes a slot 37 and channels 38 in which may be inserted or removed in close parallel proximity to xerographic plate 12 a trans- Suitable material for transparent conductive electrode 39 include glass or plastic sheets with thin evaporated metal layers or with cop per iodide coatings, but the preferred material is a sheet of glass having a transparent conductive tin oxide layer on the face nearest the xerographic plate 12 ;i.e. the lower face. This type of conductively coated glass is commercially available under the trade names Nesa and Electropane. A high voltage may be applied to conductive electrode 39 by power, supply 40.

E In a typical application of thepresent invention a xerographic plate 12 which may include a thin film 13 of vitreous photoconductive insulating selenium on'a metal base 14' is first charged toa'positive potential of several I hundred volts by applying from power supply 30, in accordance with sandard xerographic practice, a potential of several hundred volts positive to screen wires 36 and a positive potentialofabout 6000 to 8000 volts to corona generating wires 35 and. energizing motor- 29 to movecharging "bar 21 back and forth over the-.xerographic plate. Transparent conductive electrode 39, during charg- I ing by bar 21, is not in position in channels 38. Enlarger 4of FIG. 1 is then energized for a suitable length of time to project on plate 12 an image of a green separation positive of a scene or image to be reproduced. It Will'be understood that exposure could also be made to a color transparency through the use of a green filter. This first exposure produces on the plate an electrostatic latent i imagewherein those areas which have received the great- .field inthe selenium layer caused by the postive charge polarity on the surface of the xerographic plate and to make the surface negative with respect to the metal backing of the plate. No charge transfer takes place between electrode 39 and plate 12 and accordingly, the electrostatic charge'pattern on the surface'of plate 12 is not damaged or altered by application of voltage to electrode i a masked magenta image, the magenta image then being transferred to a suitable support, or the latent image may be developed with any conventional xerographic developer which is then transferred to a lithographic master to form a printing plate to be usedfor printing masked magenta of one image developer before the next succeeding color image is formed, or a separate plate may be used for each image. The three images when combined in register will form a masked color reproduction of the original scene or image.

It is alsopossi'ble to leave conductive electrode 39 and plate 12 in position-after the second exposure and to develop plate 12 by removing the high potential from electrode 39 and passing a suspension of developer pan ticles between plate 12 and electrode 39. It is further possible to make electrode 39 in the form of a wire screen orthe like in which case development of the plate maybe efiected by passing a suspension of developer particlesv through the screen and to the surface of plate 39. The teachings of patent application 'Serial No.

718,247 are pertinentv in assuringthe absence of charge transfer. Plate '12 is now exposed in register throughtransparent electrode 39 to a red separation positive of the same scene or image. This second exposure prefer ably causes a smaller change in plate potential than the first'exposure through the use of a shorter second exposure, a smaller electric field, or both. Those areas of the plate receiving the most illumination from the red sep aration will have their negative potential reduced by the greatest amount. After removal of transparent conductive electrode 39, plate 12 will then have a positive charg and a positive potential.

This potential will be greatest where the least light was received during the first exposure but will also be greatest where the most light was received during'the, second ex posure. Thus, the resulting electrostatic latent image representsa superposition of two latent images, one of them representing the original greenseparationexposure in a positive image sense and the other. representing the red separation exposure in. anegative sense. This composite image corresponds to a red masked magenta printer in photographic color print making practice. This super position, it will be noted, was achieved without exposing the xerographic plate 12 to a negative image. The plate 12 may then be removed from the apparatus and the electrostatic latent image thereon may be developed with a magenta developer by conventional techniques to form 12. In both of these methods, electrode39yfunctions as a so-called development electrode as well as fulfilling its function in forming the electrostatic'latent image. 7 I

In theabove description color separation positives I were described because the most common xerogra-phic development techniques yield a positive reproduction of, an image; that is, areas of the plate receiving the greatest illumination retain the least potential and attract the least amount of developer material. However, reversal xerographic development techniques are also common whereby the less-charged areas of a xerographic plate attract more rather than less developer than their neighbor. Where this type of development is employed the procedure described above can also be used, but color.

separation negatives rather than separation positives should be used. It will be understood, of course, that a direct exposure of the xerographic plates in a camera to an original scene through. red, green and blue filters can replace exposure to color separations where the xenographic plates have adequate sensitivities in the three spectral regions.

The invention has been described up to this point with the first or principal exposure of the xerographic plate causing the greater potential variation and the second or masking exposure causingthe lesser variation. It is, however, possibleto reverse the order of exposure, making the masking exposure first and the principal one second. When this is done, however, the potential of the resulting electrostatic latent image will vary directly rather than inversely with the intensity of illumination.

received during the principal exposure. .Accordingly, this sequence of exposure can be used where color separation negatives only are available and a conventional positiveto-positive xerographic development technique is to be used for developing the latent image. Conversely, if color separation positives are used for exposure, it may be desirable to employ a reversal development technique.

"ferred positive potential except for the relativelyshort time during which the high negative potential is applied to the adjacent transparent conductive electrode 39. It is, however, possible to make selenium xerographic plates which have equally high dark resistivity for positive or negative charging and even to make selenium plates which have higher dark resistivity for negative than for positive charging. Other types of Xerographic plates are also known which have higher dark resistivity for negative than for positive charging. As an example of this, there are presently available xerographic plates compris ing a layer of photoconductive zinc oxide powder in an insulating binder with a paper support base. Where any of these latter types of xerographic plates are used it may be desirable to initially place a negative charge on the plate and then during the second exposure to place a high positive potential on the transparent conductive electrode 39. This will result in an electrostatic latent image with a negative electric polarity which may be developed by the appropriate techniques.

A further modification of the invention utilizes xero graphic plates comprising a photoconductive insulating layer on a transparent conductive support base which may be a sheet of tin oxide coated glass. Where such a transparent backed plate is used, either or both exposures may be made through the transparent back of the xerographic plate, rather than through the transparent elec trode 39 adjacent the photoconductive insulating layer. In this case base plate 11 should be transparent or may be dispensed with entirely. 'Where both exposures are made through the back of the plate the transparency of the electrode 39 becomes immaterial, and electrode 39 may be replaced by an opaque sheet of metal or the like.

There are still further ways of carrying out the present invention in accordance with the same underlying principles and with the same results. In one such modification the xerographic plate is not initially charged, but receives the first image exposure while the adjacent electrode 39 is maintained at a high negative potential. The photoconductive insulating layer 13 becomes conductive -in illuminated areas and permits the migration of positive charge to the surface of the xero-graphic plate in response to the electric field produced by the electrode 39. The electrode is then removed, or at least the negative potential removed therefrom, leaving on the surface of the plate an electrostatic latent image of positive charge polarity in which the most strongly illuminated areas have the highest potential. The second exposure is then made without use of the electrode '39 and during this exposure plate potential is lowered in proportion to the illumination received. Thus, by this procedure also there is formed a composite electrostatic latent image which bears a positive image relation to one of the exposures and a negative or reversal relation to the other. This procedure may also be accomplished using a high positive potential on the electrode.

In a further modification the xerographic plate is first charged to a first polarity and given the first exposure without the use of the adjacent electrode, and there is then deposited on the surface of the plate a uniform increment of charge of a second polarity suflicient to reverse the sign of the potential at the surface of the plate. Since a uniform increment of charge is applied to all areas of the plate, the electrostatic latent image formed by the first exposure is not destroyed. Those areas of the plate which have a lesser potential of the first polarity now have a greater potential of the second polarity and vice versa. Suitable techniques for applying uniform increments of charge to xerographic plates are described in US. Patent 2,817,765 and applications Serial No. 556,930 and 724,164. The apparatus of FIG. 2 may be adapted to this technique by replacing charging bar 21 with one of the devices shown in the above referenced applications. After the polarity of the plate potential has been reversed, the second exposure is latent image which bears an opposite relation to the two exposures.

The distinguishing feature common to all embodiments of the invention is the fact that the first exposure of the xerographic plate is made while there is an electric field of one polarity across the photoconductive insulating layer, and the second exposure is made, while preserving the latent image formed by the first exposure and with an electric field of opposite polarity across the photoconductive insulating layer. As can be seen from the various embodiments, these fields can be produced in the photoconductive insulating layer either by depositing charge on the surface of the layer, by pulling charge through the layer to the surface, or by placing a high potential electrode adjacent to the layer.

While the present invention has been described in terms of its usefulness in the art of color print making in xerography, it is to-be understood that it can also be used wherever it is desired to form a single xerographic electrostatic latent image from two successive exposures in such a way that one exposure raises the potential of the final latent image while the other exposure reduces it.

What is claimed is:

The method of xerographic color masking in which there is formed a superposed composite xerographic latent charge pattern on a xerographic plate including a photoconductive insulating surface layer from two separate image exposures each being to a difierent primary color component of an original color image comprising charging the photoconductive insulating layer to a first polarity, exposing the photoconductive insulating layer to a first primary color component of an original thereby forming a latent charge pattern of said first polarity wherein the amount of charge remaining on the photoconductive insulating layer is inversely proportional to the amount of light received during exposure to said first primary color component, applying uniform increments per unit area of charge of a second polarity opposite to said first polarity to the surface of the photoconductive insulating layer bearing said latent charge pattern from an adjacent corona discharge device adapted to deposit charge at a rate substantially independent of the potential on said photoconductive insulating layer thereby everywhere reversing the polarity of the photoconductive insulating layer to said second polarity and charging everywhere to a sensitizing potential while maintaining the charge variations produced by said first exposure, and thereafter and as a separate and distinct operation exposing the photoconductive insulating layer in register to a second primary color component of said original image thereby forming a latent charge pattern of said second polarity in which the amount of charge varies directly with the amount of light received during the first exposure and indirectly with the amount of light received during the second exposure, one of the exposures being greater and producing more charge variation than the other.

References Cited in the file of this patent UNITED STATES PATENTS 2,297,691 Carlson Oct. 6, 1942 2,808,328 Jacob Oct. 1, 1957 2,817,765 Hayford et a1. Dec. 24, 1957 2,833,648 Walkup May 6, 1958 

